Systems on Chip (SOC) typically comprise a multitude of separate sub-systems, which are integrated in the same Application-Specific Integrated Circuit (ASIC). Additionally, some or all the sub-systems may connect to modules which are external to the ASIC, through ASIC input/output ports. The input/output ports may sometimes be multiplexed on the SOC pads.
Techniques for multiplexing General Purpose IO (GPIO) are described, for example, in “SFF Committee SFF-8485 Specification for Serial GPIO (SGPIO) Bus,” Revision 0.7, Feb. 1, 2006, which is incorporated herein by reference.